The image forming apparatuses, such as copiers, printers, and facsimiles, applying electrophotographic technology, may encounter image density variation of a toner image due to individually different characters of apparatuses, variation over time, changes of conditions surrounding the apparatuses which include temperature, moisture, and the like. Heretofore, a variety of technologies have been proposed for the purpose of ensuring a stable image. Among these technologies, for example, is technology in which a small test image (patch image) is formed on an image carrier and the operating conditions are optimized based on the density of the patch image. In this technology, in order to ensure a stable image quality, the operating conditions are optimized at a predetermined timing. For example, in order to minimize density fluctuation stemming from toner property variation over time, the image forming apparatus described in the patent document #1 memorizes the parameters that indicate the status of toners in a developer and executes a readjustment of operating conditions every time when a parameter reaches a predetermined threshold value.
Patent Document 1: Japanese Patent Application Laid-Open Gazette No. 2004-177928